Beta-VLDL metabolism by pigeon macrophages. Evidence for two binding sites with different potentials promoting cholesterol accumulation.

Previous studies from our laboratory (J Lipid Res 1988;29:643-656) have shown that thioglycolate-elicited peritoneal macrophages from White Carneau and Show Racer pigeons, like mammalian macrophages, have on their surfaces specific receptors for acetylated low density lipoprotein (acLDL) and beta-migrating very low density lipoproteins (beta-VLDL). The binding kinetics of beta-VLDL were complex, however, suggesting more than one binding site. The purpose of the present study was to further characterize these beta-VLDL binding sites. Scatchard analysis of 125I-beta-VLDL binding curves indicated at least two classes of binding sites. The first binds pigeon beta-VLDL and LDL with high affinity (Kd approximately 7 micrograms/ml), is down-regulated by cholesterol loading, requires calcium, and is destroyed by the proteolytic enzyme, pronase. This pigeon beta-VLDL receptor is specific for pigeon beta-VLDL and LDL and does not recognize HDL, acLDL, methyl LDL, cynomolgus monkey LDL, or rabbit beta-VLDL. Like the mammalian macrophage beta-VLDL receptor, the "pigeon beta-VLDL receptor" has many of the characteristics of an LDL receptor. The second class of binding sites is relatively nonspecific, recognizing both pigeon and rabbit beta-VLDL, LDL, acLDL, methyl LDL, and HDL. Binding to this site is not altered by incubation of macrophages with pronase or by cholesterol loading. This binding site has low affinity for beta-VLDL (Kd approximately 100 micrograms/ml), but high capacity. We have called this the "lipoprotein binding site," a term used by others to describe similar lipoprotein binding characteristics on a variety of cells. Not only does binding to this site promote the internalization and degradation of lipoproteins, but it may also facilitate the independent uptake of cholesterol. This conclusion is based on the observation that more cholesterol accumulates in cells incubated with rabbit beta-VLDL, which binds only to the lipoprotein binding site, than can be accounted for by beta-VLDL uptake and degradation. Since the lipoprotein binding site recognizes a variety of normal, as well as abnormal, lipoproteins, it would not require the generation of abnormal lipoprotein products, as must occur with the scavenger receptor, to promote the accumulation of cholesteryl esters in macrophages of atherosclerotic lesions. This, coupled with the fact that the lipoprotein binding site is not down-regulated by cholesterol loading, suggests that it could provide an alternative mechanism to the scavenger receptor pathway for the formation of foam cells.